Markovnikov's Rule

The halide component of HX bonds preferentially at the more highly
substituted carbon, whereas the hydrogen prefers the carbon which
already contains more hydrogens.

Anti-Markovnikov

Some
reactions do not follow Markovnikov's Rule, and
anti-Markovnikov products are isolated. This is a feature for
example of radical induced additions of HX and of
Hydroboration.

Mechanism

The proton adds first to the carbon-carbon double bond. The carbon bearing
more substituents forms a more stable carbenium ion; attack of bromide ion
follows in a second step:

Markovnikov

Radical reactions require an initiation step. In this example, a bromo
radical is formed.

The reversal of the regiochemistry of addition is the result of the reversal
of the order in which the two components add to the alkene. Radical addition
leads to the formation of the more stable radical, which reacts with HBr to give
product and a new bromo radical: